Cold-cathode gas-discharge tubes



Jan. 5, 1960 PULSE AMPLITUDE IN VOLTS.

R. R- BACK COLD-CATHODE GAS-DISCHARGE TUBES Filed Aug. 14, 1956 PULSEDURATION IN -MICROSECS.

irncs United States Patent signor to Hivac Limited, South Ruislip,England, a

British company Application August 14, 1956, Serial No. 603,891

Claims priority, application Great Britain August 25, 1955 3 Claims.(Cl. 313-197) This invention relates to cold-cathode gas-discharge tubesand is more particularly concerned with tubes of the so-called relaytype in which the main discharge between a cathode and an anode can beinitiated by a control potential applied to an associated triggerelectrode. It is a characteristic of such tubes that, once the maindischarge is initiated, the trigger electrode has no further controluntil the potential between the cathode and anode of the main dischargegap is suitably reduced.

In such tubes there are a number of important parameters which governthe utility of the tube in specific circuit applications. Among theseparameters are: (a) the main gap breakdown voltage which is the value ofthe voltage necessary between anode and cathode of the main dischargegap to cause self-breakdown, i.e. without the aid of a control potentialon the trigger electrode; (b) the trigger breakdown voltage which is theminimum value of the voltage between the trigger electrode and anotherelectrode of the tube (usually the main discharge cathode) necessary tocause the initiation of the required main discharge; (c) the main gapmaintaining voltage which is the minimum value of the voltage betweenthe anode and cathode of the main discharge gap necessary to maintainthe main discharge with a given value of discharge current flowing; (d)the dynamic trigger sensitivity value which is a measure of the timeperiod between the instant of application of a suitable controlpotential to the trigger electrode and the instant of initiation of theconsequential main discharge, and (e) the extinguishing or recovery timewhich is a measure of the time period between the instant of reductionof the voltage existing between the main anode and main main gapmaintaining voltage and the subsequent instant when the triggerelectrode regains control of the tube operation. Other important factorsin the commercial design, manufacture and use of such tubes are: (i) thetube-to-tube variation of the above parameters occurring in a batch ofotherwise identical tubes; (ii) the main discharge current rating whichis the maximum permissible value of the main discharge current and (iii)the physical size of the tube.

One object of the present invention is to provide an improved form ofsuch relay tube construction partic ularly adapted for use inapplications such as electronic counting circuits and telephoneswitching systems. An-

other object is to provide a tube construction in which the tube-totubespread orvariation of parameters such cathode to a'value below that ofthe above-mentioned 2 annular rim which merges at its inner edgesmoothly into the end of said tube, the anode is arranged to be of rodor wire-like form disposed to lie coaxial with said cathode tube butsubstantially displaced away from the end of said cathode tube providedwith said annular rim while the trigger electrode is arranged to be ofring-like form located around and in close spaced relationship to theouter edge of the aforesaid annular rim of the cathode.

In order that the invention may be more readily understood a number ofconstructional embodiments thereof will now be described with referenceto the accompanying drawings, in which:

Fig. l is a side elevational view of one form of tube construction, theenvelope and other parts being shown partly in section.

Fig. 2 is a fragmentary side view of the electrode structure taken in adirection at right angles to that of Fig. 1. P Fig. 3 is a plan view ofthe electrode structure of Fig. 4 is a fragmentary part-sectional,part-elevational view of one alternative form of cathode construction.

Fig. 5 is a fragmentary part-sectional, part-elevational View of anotheralternative form of cathode construction, w ile Fig. 6 is a graphillustrating a certain characteristic of the tube as shown in Figs. 1, 2and 3.

Referring first to Figs. 1, 2 and 3, the tube shown comprises a cathode10, an anode 11 and a trigger electrode 12 located within a tubularshaped glass envelope 13.

The cathode 10 is in the form of a cylindrical tube 15 provided with anannular rim 14 projecting radially outwards from its lower end, theinner edge of such rim merging smoothly into the main tubular portion15. The upper end of the cathode cylinder may be inturned slightly asshown to increase its stiffness. I

The tubular portion 15 is disposed with its axis coin cident with thatof the related anode 11 which is in the form of a rod or wire 16 locatedwithin the bore of a recess 17 formed in a stem 18 of glass which,although initially separate, is subsequently fused into integralrelationship with the envelope 13. As will be seen from the drawing, theend of the anode nearest to the cathode is displaced a substantialdistance away from the end of such cathode provided with the annular riml4.

The stem 18 has a platform or button 19 integral with its upper end, thebore of the recess 17 passing through said button and terminating at itsupper surface. The button 19 serves as an anchorage for a number ofsupport wires which are fused therein, such support wires includng twowires 20 for supporting the cathode 10 and two turther wires 21 whichsupport the trigger electrode 12.

The trigger electrode 12 is of ring-like form and compr ses a short,nearly complete,'cylinder 22 disposed coaxial with the cathode 10. Theinner radius dimension of the cylinder 22 is only slightly greater thanthat of the I .outer edge of the rim 14 so as to provide a narrowannular trigger-to-cathode discharge gap. The upper edge of the cylinder22 is preferably so disposed that it lies approximately in themid-thickness plane of the annular rim 14 of the cathode.

The inner surface of the cathode 10 is coated with suitable emissivematerial having a low work function from a level as shown at 23 on thetubular portion 15 down to and around the under or anode-facing surfaceof the rim 14. The cathode is secured in position by attachment to atransverse mounting plate 26 whose opposite ends are connected, as byspot welding, to the support wires 20. p

The enclosing glass envelope 13 is of tubular form and is constructedwith a pinch region 24 at its lower end through which the variousconnecting leads 25, 26, 27

from the cathode 10, the anode 11 and the trigger electrode 12respectively pass outwardly in the usual way. A gettering member 28 islocated in the tube space below the button 19, said gettering memberbeing secured to one of the connecting wires.

The gas filling usedfor the tube may be any suitable combination of raregases, chosen and adjusted in their proportions and filling pressure inaccordance with well known. practice, to suit the particular type oftube characteristics required. A preferred gas filling for manypurposes, including that of a discharge tube suitable for electroniccounter operation, is one embodying neon and argon, for example, amixture of 99%. neon and 1% argon. With the latter gas a convenientfilling pressure is 40 mm. of mercury. Other gaseous mixtures may.however, be helium and argon or, a combination of neon, helium andargon.

An alternative form of cathode construction is shown in Pig. 4. Thisalso has a cylindrical shaped main tubular portion 15a but the lowerannular projecting rim 14a is made of approximately semi-circularcross-section. with the inner edge merging smoothly into the maintubular portion as before. As in the example shown in Figs. 1, 2 and 3,the under-surface of this curved rim is coated with emissive materialcontiguously with the lower surface portion of the inside of thecylinder.

A further alternative form of cathode construction is shown in Fig.where the main portion is in the form of a hollow truncated cone 15b.The lower edge of this cone is shown merging into a rim 14b ofsemi-circular cross-section like that shown in Fig. 4 but such outwardlyprojecting rim may be of other broadly similar shape, for instance,substantially of flat annular form as shown in Figs. 1, 2 and 3.

The trigger electrode 12 may also be modified in various ways withoutdeparting from the scope of the invention. It may, for example, bearranged in the form of two C-shaped elements disposed in diametricallyopposed relationship one on each side of the cathode. Alternatively, andin some cases preferably, the trigger electrode may be in the form of acomplete ring but this can lead to difiiculty during manufacture byabsorbing an undue amount of the RF. energy used for heating the cathodeduring processing of the tube in its manufacture. The trigger electrodemay also be in the form of a member made of circular section wireinstead of flat metal strip as shown.

The arrangement according to the invention emplo ing a short tubularcathode with an outwardly directed rim in conjunction with an anodedisplaced axially away the rim end of the cathode and with the triggerelectrode immediately surrounding the outer rim of the cathode providesa construction in which the anode can always see the initialtrigger-to-cathode discharge and serves also to provide an arrangementin which the dynamic trigger sensitivity value is high and in which,moreover, the spread of the various tube parameters as previouslydiscussed is materially reduced.

Fig. 6 illustrates, by way of example, the improve ment obtained for thedynamic trigger sensitivity value in a tube according to the invention(full line curve) as compared with a closely equivalent tube ofsubstantially equivalent rating (dotted line curve). Thus, whereas withthe existing tube using an 80 volt triggering pulse, the triggering timewas of the order of 225 microseconds, that of a tube according to theinvention was only 55 microseconds. In addition, whereas the spread ofthe parameter (b) given above was from 65-85 volts, the equivalentfigures for a tube. according to the invention were of the order of 6274volts only. The parameter (0) given above showed a spread of 6878 voltsfor a tube according to the invention as compared with a spread of 69-85volts for the existing type of tube. The spread of the dynamic triggersensitivity parameter (d) was between 35 and 90 m croseconds for a tubeaccording to the invention as compared. with between 58 and 450 micro,-

seconds for the existing tube. Similarly, the parameter (e) discussedabove showed a variation of 400-700 microseconds for a tube according tothe invention as compared with a variation of 300-2000 microseconds forthe existing tube. Atthesame time, the main gap break-down voltageparameter (a) was increased from 180 volts minimum for the existing tubeto 210 volts minimum for the tube according to the invention.

In the particular, sub-miniature, tube according to the invention. heoverall bulb size was approximately'35 mm. length by 10.5 mm. diameterand the cathode to anode. spacing dimension a, Fig. l, was 5.75 mm. Theanode recession dimension b, was approximately 2 mm. The tube currentrating was 1 ma.

I claim:

1. A cold-cathode gaseous discharge. tube of the relay type comprising acathode, an anode and a separate trigger electrode by which a dischargebetween said cathode and an n b init at by h p a i n. of Su t bl controlpotential thereto, said cathode having the. shape of a short metal tubeprovided at one end with an outwardly directed integral annular rimwhich merges into the end of said tube with a curve of appreciableradius, said cathode having an electron-emissive surface coating uponthe inner surface of said tube and extending around the contiguoussurface of said flange, said anode being in the form of a rod disposedto lie coaxial with the axis of said cathode tube and sunk within aninsulating recess with the end of said red at a position substantiallydisplaced away from but facing towards that end of said cathode which isprovided with said annular rim and said trigger electrode having theform of a short length cylinder of internal diameter slightly greaterthan the peripheral diameterof said rim and being disposed coaxial withthe axis of said cathode and located immediately around and in closespaced relationship to the outer edge of the aforesaid annular rim ofthe cathode with the greater part of the length of said triggerelectrode cylinder extending from said rim towards said anode.

2. A cold-cathode gaseous discharge tube of the relay type comprising acathode, an anode and a separate trigger electrode by which a dischargebetween said cathode and anode, can be initiated by the application of asuitable control potential thereto, said cathode having the shape of ashort metal tube provided at one end with an outwardly directed integralannular rim which merges into the end of said tube with a curve ofappreciable radius, said cathode having an electron-emissive surfacecoating upon the inner surface of said tube and extending around thecontiguous surface of said flange, said anode being in the form of a roddisposed to lie coaxial with the axis of said cathode tube at a positionsubstantially displaced away from but facing towards that end of saidcathode which is provided with said annular rim and said triggerelectrode having the form of a short length axially gapped cylinder ofinternal diameter slightly greater than the peripheral diameter of saidrim and being disposed coaxial with the axis of said cathode and locatedimmediately around and in close spaced relationship to the outer edge ofthe aforesaid annular rim of the cathode with the greater part of thelength of said trigger electrode cylinder extending from said rimtowards said anode.

3. A cold-cathode gaseous discharge tube of the relay type comprising acathode, an anode and a separate trigger electrode by which a dischargebetween said cathode and anode can be initiated by the application of asuitable control potential thereto, said cathode having the shape of ashort metal tube provided at one end with an outwardly directed integralannular rim which merges into the end of said tube with a curve ofappreciable radius, said cathode having an electron-emissive surfacecoating upon the inner surface of said tube and extending around thecontiguous surface of said flange, said anode being in the form of a roddisposed to lie coaxial with the axis Qt said. athode ube a a Past iessubstantia ly diseased away from but facing towards that end of saidcathode which is provided with said annular rim and said triggerelectrode having the form of two electrically interconnected C-shapcdhalves of a short length cylinder disposed coaxial with the axis of saidcathode and located immediately around and in close spaced relationshipto the outer edge .of the aforesaid annular rim 'of the cathode with thegreater part of the length of each of said halfcylinders extending fromsaid rim towards said anode.

References Cited in the file of this patent UNITED STATES PATENTSMendenhall Nov. 9, 1937 Ekstrand .i Feb. 3, 1942 Martin Sept. 7, 1943Ingram Oct. 12, 1943 Slack et al. Mar. 23, 1948 Stutsrnan July 29, 1952

